Postgres convert string to time

I have the following query:

SELECT id, start_date::TIME, occurrence->0->>'startsOn'  FROM service WHERE name='A.F';

Which return:

id  | start_date |            ?column?             
------+------------+---------------------------------
 1573 | 18:00:00   | Mon, 29 Jun 2015 18:00:00 +0000
 1592 | 10:00:00   | Wed, 24 Jun 2015 10:00:00 +0000
 1605 | 18:00:00   | Thu, 25 Jun 2015 18:00:00 +0000
 1571 | 10:00:00   | Mon, 29 Jun 2015 10:00:00 +0000
 1591 | 20:15:00   | Tue, 30 Jun 2015 20:15:00 +0000
 1578 | 18:00:00   | Mon, 29 Jun 2015 20:00:00 +0000
 1620 | 12:00:00   | Sat, 27 Jun 2015 12:00:00 +0000
(7 rows)

what I am trying to do is convert occurrence->0->>'startsOn' to time, so the expected result should be:

  id  | start_date |            ?column?             
------+------------+---------------------------------
 1573 | 18:00:00   | 18:00:00
 1592 | 10:00:00   | 10:00:00
 1605 | 18:00:00   | 18:00:00
 1571 | 10:00:00   | 10:00:00
 1591 | 20:15:00   | 20:15:00
 1578 | 18:00:00   | 20:00:00
 1620 | 12:00:00   | 12:00:00

i tried the following:

SELECT id, start_date::TIME, occurrence->0->>'startsOn'::TIME  FROM service WHERE name='A.F';

But it is not working as it gives me the following syntax error:
ERROR:  invalid input syntax for type time: "startsOn"